130 Science of Plant Life 



in the stem are each year farther removed from the corre- 

 sponding annual layers of the bark. 



Annual monocots increase in thickness through the enlarge- 

 ment of the bundles and by the multipUcation and enlarge- 

 ment of the pith cells. Perennial monocots, like the bamboo 

 and asparagus, have underground stems to which new and 

 thicker stem segments are added each year. The aerial, erect 

 branches never increase in size after they are once mature ; 

 but the erect branches from old underground stems are from 

 the beginning much thicker than those from young plants. 

 Consequently, no little bamboo rod could ever grow into a 

 bamboo beam. No large bamboo beam was ever a slender 

 rod. These aerial branches come out of the ground nearly 

 as thick as they will be when mature. Asparagus plants are 

 several years old before the underground stems send up thick, 

 upright branches suitable for marketing. 



Heartwood and sapwood. As the trunks of trees increase 

 in thickness, all the living cells toward the center of the stem 

 gradually die. The wood usually changes in color after the 

 death of these cells. In a peach tree only the outer three or 

 four annual rings may be alive. In a walnut trunk 2 feet in 

 diameter, all but the outer 2 inches may be dead. The dead 

 wood still helps to support the enormous weight of the tree 

 top, but it has nothing to do with the conduction of water and 

 substances in solution. This inner dead wood is called the 

 heartwood; the outer living wood is called the sapwood. The 

 heartwood in many species of trees is much more valuable 

 than the sapwood for lumber, because of its color and greater 

 durability. 



Stems in relation to gravity. The direction of growth of 

 stems is for the most part determined by gravity. The erect 



